1. Field of the Invention
The present invention relates to a method of producing a semiconductor device by use of a precursor of an organic semiconductor compound.
2. Description of the Related Art
The development of thin-film transistors employing an organic semiconductor gradually began to become more active in the latter half of the 1980s. In recent years the basic performance of thin-film transistors employing an organic semiconductor now exceeds the basic performance of thin-film transistors employing amorphous silicon. There are often cases where organic semiconductor materials have a high affinity with a plastic substrate on which a semiconductor device, such as a thin-film field effect transistor (FET), is formed. Therefore, organic semiconductor materials are desirable as the material for a semiconductor layer in devices which are required to have flexibility or light weight. Further, it is possible for a part of the organic semiconductor material to form a film by application of a solution or by use of a printing method. If such a material is used, a device with a large area can be fabricated at low cost.
Examples of organic semiconductor materials which have hitherto been proposed include the following. First, there are included acenes disclosed in Japanese Patent Application Laid-Open No. H05-55568 such as pentacene and tetracene; low-molecular compounds disclosed in Japanese Patent Application Laid-Open No. H05-190877 such as phthalocyanines including lead phthalocyanine, perylene and tetracarboxylic acid derivatives thereof; and high-molecular compounds disclosed in Japanese Patent Application Laid-Open No. H08-264805, such as aromatic oligomers typified by thiophene hexamers referred to as α-thienyl or sexythiophene, as well as polythiophene, polythienylenevinylene and poly-p-phenylenevinylene. Incidentally, many of these compounds are also disclosed in Advanced Material, Vol. 2, pp. 99-117, 2002.
The characteristics such as non-linear optical characteristics, electrical conductivity and semiconductor characteristics, which are required when producing a device with the above compounds being used for a semiconductor layer, do not depend only on the purity of a material but also largely depend on the crystallinity and orientation of the material.
Meanwhile, many of low-molecular compounds having a π-conjugated system extended (e.g., pentacene) have high crystallinity and are insoluble in solvents. Therefore, thin-films of most of these compounds are generally formed by using a vacuum deposition process. While it is known that pentacene exhibits a high electric field effect mobility, there has been a problem that pentacene is unstable in the atmosphere, easily oxidized and susceptible to degradation. Moreover, in the case of using a vacuum film forming technique such as vacuum-deposition process, the merits of the organic semiconductor material in enabling production at low cost are lost.
On the other hand, there are often cases where organic semiconductors using a π-conjugated polymer are able to easily form a thin film using a solution coating process or a similar technique. Therefore, applied development of organic semiconductor films using a π-conjugated polymer has been progressing due to the fact that such films often have excellent formability (“Japanese Journal of Applied Physics”, Japan Society of Applied Physics, Vol. 30, pp. 596-598, 1991). In the case of π-conjugated polymers, it is known that the state of arrangement of molecular chains thereof has a large influence on the electrical conductivity. Similarly, it has been reported that the electric field effect mobility of π-conjugated polymer field effect transistors largely depends on the state of arrangement of molecular chains in a semiconductor layer (“Nature”, Nature Publishing Group, Vol. 401, pp. 685-687, 1999). However, since the arrangement of molecular chains of π-conjugated polymers is effected in a period from coating of a solution to its drying, there is a possibility that the state of arrangement of molecular chains may vary greatly depending on changes in environment or a kind of coating methods. Thus, there is a concern that the electric field effect mobility varies depending on coating conditions, which makes stable production difficult.
In recent years there has also been reported a FET which employs a film in which a thin film made of a soluble precursor of pentacene is formed by coating, which is then converted to pentacene by heat-treatment or light irradiation (“J. Appl. Phys.”, Vol. 79, p. 2136, 1996; WO A1 2004079834; Japanese Patent Application Laid-Open No. 2004-221318). In the case of converting the precursor to pentacene by heat-treatment, there have been the problems that a high-temperature treatment is required for the conversion to pentacene, and that eliminated components with a large mass need to be removed by pressure reduction. On the other hand, in the case of converting the precursor to pentacene by light irradiation, while a high-temperature treatment is not required, there still remains the problem that obtained pentacene is unstable in the atmosphere, easily oxidized and susceptible to degradation.
As described above, hitherto, when producing a semiconductor device using an organic semiconductor compound such as a field effect transistor, a semiconductor layer has been formed having good crystallinity and orientation by undergoing a vapor deposition step or the like. However, there have been the problems that vapor deposition is expensive, and acenes, which are typical organic semiconductors, are susceptible to be oxidized to be thereby degraded. Further, while coating methods are low cost, because the arrangement state of molecular chains can be disrupted unless the coating conditions are strictly controlled, the variability in characteristics between individual devices has been a concern.